This invention relates to optical functional devices, their manufacturing method and an optical communication system. More specifically, the invention relates to optical functional devices including distributed feedback (DFB) lasers with an optical waveguide structure having 2nd- or higher-order gratings, and other various optical functional devices having similar waveguide structures, and their manufacturing methods. The invention also relates to an optical communication system using these devices.
DFB lasers are often used in optical communication because single longitudinal mode oscillation is easily obtained. Single longitudinal mode oscillation is realized by a periodic structure formed along the waveguide of a laser, that is, diffraction gratings, because optical feedback for resonance becomes greatest in a specific longitudinal mode determined by the period of the gratings.
In optical communication using silica fibers, both the 1300 nm wavelength band and the 1550 nm wavelength band are used because these wavelength bands correspond to low-loss, low-dispersion regions of silica fibers. The InGaAsP/InP material system are most suitable for fabricating light emitting devices which emit in these bands. Therefore, InGaAsP/InP DFB lasers are widely used for optical communication.
FIG. 19 is a longitudinal cross-sectional view showing a structure of a conventional InGaAsP/InP-DFB laser. That is, FIG. 19 shows a cross-sectional view taken along a plane parallel to the waveguide of the DFB laser. This laser has sing 1st-order Bragg gratings with a xcex/4 phase shift. The structure of the laser shown here is explained below, following to its manufacturing procedures.
First made on an n-type InP substrate 101 is an n-type InP buffer layer 101xe2x80x2 by crystal growth. Next grown thereon are an active layer 102 having a multi-layered structure of InGaAsP quantum well layers and barrier layers, and a waveguide layer 103 having a lower refractive index than that of the active layer 102. After these steps of growth, the wafer is taken out from the growth furnace.
After that, 1st-order gratings 110 are grooved on the waveguide layer 103. In this process, a phase shift 115 by xc2xc or xe2x88x92xc2xc of the wavelength xcex in the waveguide, is simultaneously made at a central position of the cavity. The same effect is also obtained by using a structure changing the effective refractive index of the waveguide instead of the actual phase shift. That is, even when the period of the gratings 110 is uniform, a region (not shown) where the waveguide structure changes in width, thickness or refractive index effectively functions as a phase shift.
After that, while keeping the configuration of the gratings 110 and the phase shift 115, a p-type InP cladding layer 104 and InGaAs contact layer 105 are stacked on them by crystal growth.
Thereafter, a stripe structure (not shown) is made to extend in parallel to the wafer surface. Typical stripe structures are BH structure (buried heterostructure) and RWG (ridge waveguide) structure.
After that, a p-side electrode is formed on the p-type contact layer 105, and an n-side electrode is formed on the bottom surface of the n-type substrate 101 (both not shown).
In the phase shift structure, the probability of the single longitudinal mode operation decreases if the reflectivity of both edges exceeds 1%. Therefore, both edges are coated by AR (anti-reflection) coating 150. This can be realized by depositing dielectric thin films on the edges by the thickness of xcex/4 (xcex: oscillation wavelength).
Other than the structure shown in FIG. 19, there is a HR/AR (high reflectivity/anti-reflection) structure. A cross-sectional configuration of a laser having this structure is shown in FIG. 20. A difference from FIG. 19 lies in having no phase shift 115 but having a HR coat 160 with the reflectivity greater than 90% on one of the edges. The HR coat 160 is a dielectric multi-layered film. It is discerned here that the relative phase between the HR coat edge and the gratings corresponds to the phase shift of FIG. 190, if it is folded back about the position of the HR coat 160 as the center of a mirror image. Usually, the phase of the gratings at the edge cannot be controlled. Therefore, the probability of obtaining preferable facet phases becomes lower. Taking account of it, the yield of the single longitudinal mode in the structure of FIG. 20 is inferior to that of the xcex/4 for xcfx80/2 shift structure of FIG. 19. Nevertheless, because of a large optical output from the AR edge, it is still useful as a high-output or high-efficiency structure.
These conventional DFB lasers, however, involved the problem that they were difficult to manufacture and often difficult to realize acceptable properties.
More specifically, the period of the gratings of a DFB laser utilizing 1st-order Bragg diffraction has to be approximately 200 nm to realize the wavelength of 1300 nm and approximately 240 nm to realize the wavelength of 1550 nm. When making the gratings, patterning must be as small as half the period, and an ultimate nano-process technique is required. Therefore, it is not easy to realize such gratings.
On the other hand, coupling efficiency K, which strongly affects the performance DFB laser, depends on the shape of the gratings. If the coupling efficiency K is excessively small, sufficient distribution feedback is not obtained, and the laser becomes difficult to oscillate in a single longitudinal mode. If it is excessively large, the threshold current of other longitudinal modes also become lower, and spatial hole burning phenomenon caused by longitudinal non uniformity of optical power makes single longitudinal mode operation unstable. That is, the coupling efficiency K must be within an optimum range. (Since the property of a DFB laser depends on its cavity length L as well, it is usually evaluated in terms of kL by multiplying L.) In order to realize an optimum value of K, the gratings must be precisely fabricated in depth and configuration. However, considering that 1st-order gratings are extremely fine as explained above, control of their configuration is very difficult. Additionally, optimum depth of 1st-order gratings is as very shallow as 20 to 30 nm approximately, its control is also difficult. As a result, there has been the problem that an optimum K value cannot be realized, and lasers satisfying desired properties cannot be obtained easily.
However, if utilizing 2nd- or higher-order Bragg gratings, their period is elongated to twice or more than 1st-order gratings, and the size of their patterning is enlarged sufficiently to make their fabrication easy. Additionally, depth of the gratings for obtaining the same value of K increases as well, and this makes it easy to control K. 
However, the use of 2nd- or higher-order gratings introduces lower-order diffraction light as radiation modes emitted from the waveguide. This is a loss for the DFB laser. This increases the threshold currents and deteriorates single longitudinal mode capability.
The present invention has been made from recognition of these issues. It is therefore an object of the invention to provide optical functional devices, such as low-threshold DFB laser, which are decreased in radiation mode loss even when using easily processed high-order gratings. It is a further object of the invention to improve their single longitudinal mode properties higher than those of using 1st-order gratings. It is another object of the invention to provide an optical functional device as surface-emitting laser (GCSEL: grating-coupled surface emitting laser) using 2nd-order gratings which can be optimized in threshold currents and emitted output or in light emitting pattern. Additionally, it is an object of the invention to provide their manufacturing method and an optical communication system using these devices.
According to the invention, there is provided an optical functional device including a waveguide and gratings formed along the waveguide for emitting light at a specific wavelength, comprising: the diffraction gratings causing 2nd- or higher-order Bragg diffraction at the gratings having a unit structure which is asymmetric in the direction along the waveguide, and the gratings having a phase shift.
According to the invention, there is further provided an optical functional device including a waveguide and gratings formed along the waveguide for emitting at a specific wavelength, comprising: the diffraction gratings causing 2nd- or higher-order Bragg diffraction at the gratings having a unit structure which is asymmetric in the direction along the waveguide, and the gratings varying in degree of the asymmetry along the waveguide direction.
According to the invention, there is further provided an optical functional device including a waveguide and gratings formed along the waveguide for emitting light at a specific wavelength, comprising: the diffraction gratings causing 2nd- or higher-order Bragg diffraction, the gratings having a unit structure which is asymmetric in the direction along the waveguide; and the gratings having a first part including unit structures each having a first asymmetry, and a second part including unit structures each having a second asymmetry different from that of the first part.
The gratings may have the first part starting from one end of the waveguide and the second part from the other, the gratings having a phase shift between the first part and the second part.
The first part and the second part may be so configured that net radiation power escaped from the waveguide through the gratings attenuate interactively.
The gratings may include a plurality of protrusions periodically aligned along the waveguide and each forming each unit structure, each protrusion defining a slope facing to the phase shift being gentler than the slope on the far side of the shift, the phase shift of {nxcexxc2x1(xe2x85x9xcx9cxe2x85x9c)}xcex where xcex is the guided wavelength, and n is an arbitrary integer.
Alternatively, the first part and the second part are so configured that radiation mode power escaped from the waveguide through the gratings interactively intensify.
The gratings may include a plurality of protrusions periodically aligned along the waveguide and each forming each unit structure, each protrusion defining a slope facing to the phase shift being steeper than the slope on the far side of the shift, the phase shift of {nxcexxc2x1(xe2x85x9xcx9cxe2x85x9c)}xcex is a guided wavelength in the waveguide, and n is an arbitrary integer.
According to the invention, there is further provided an optical functional device including a waveguide and gratings formed along the waveguide for emitting at a specific wavelength, comprising: the diffraction gratings causing 2nd- or higher-order Bragg diffraction, the gratings having a unit structure which is asymmetric in the direction along the waveguide; and the waveguide having a high-reflectivity facet having a high optical reflectivity at one end thereof, and a low-reflectivity facet having a low optical reflectivity at the other end thereof.
The reflectivity of the high-reflectivity facet is preferably not less than 60%, and the reflectivity of the low-reflectivity facet is preferably not higher than 1%.
The gratings and the relative phase at the high reflectively facet may be so configured that radiation mode power escaped from the waveguide therethrough attenuate interactively.
The gratings may include a plurality of protrusions periodically aligned along the waveguide and each forming each unit structure, each protrusion defining a slope facing to the high-reflectivity facet being gentler than a slope on the other side, the relative phase at the high-reflectivity facet being nxcex+(3xcex/4-xcex/8xcx9c+xcex/8 where xcex is the guided wavelength, and n is an arbitrary integer. That is, here is provided an offset amount in the range from xe2x85x9 to xe2x85x9c from arbitrary integer multiplied by the guided wavelength.
Alternatively, the gratings and the relative phase at the high-reflectivity facet may be so configured that radiation mode power escaped from the waveguide therethrough intensify interactively.
The gratings may include a plurality of protrusions periodically aligned along the waveguide and each forming each unit structure, each protrusion defining a slope facing to the high-reflectivity facet being steeper than a slope on the other side, the relative phase at the high-reflectivity facet being 2nxcfx80+3xcfx80/2-xcfx80/2xcx9c3/2xcfx80 guided, and n is an arbitrary integer. That is, here is provided an offset amount in the range from xcex/8 to 3xcex/8 from arbitrary integer times of the guided wavelength.
The waveguide may be formed by processing a thin film into the form of a stripe, and the gratings may be formed on a side plane of the stripe-shaped said thin film.
The optical functional device may function as a DFB laser or a DBR laser.
According to the invention, there is further provided an optical functional device manufacturing method comprising the steps of: forming a mask having a pattern of gratings on a waveguide material;
screening a part of the mask; processing the other part of the pattern not screened by the mask by anisotropic dry etching which can control asymmetry of gratings grooved into the waveguide material by controlling the incident direction; and screening the part of the mask heretofore not screened by the mask, and opening the part of the mask heretofore screened by the mask, and conducting anisotropic dry etching from an incident angle different from that of the former dry etching.
According to the invention, there is further provided an optical functional device manufacturing method comprising the steps of: forming a mask having first slopes and second slopes approximately symmetric with the first slopes on a waveguide material; and conducting etching having an anisotropy from a direction substantially parallel with the second slopes, asymmetric gratings being grooved into the waveguide material in accordance with the ratio in etching speed between a material forming the mask and the waveguide material.
According to the invention, there is further provided an optical communication system comprising an optical functional unit obtaining a light signal output and an optical fiber transmitting said light signal output by said optical functional unit, said optical functional unit including any of the optical functional devices summarized above.
The invention embodied and used in the above-summarized modes performs the following effects.
First of all, the invention realizes inexpensive, high-performance optical functional devices using higher-order gratings which are easy to process.
That is, by controlling the blaze angle or other structural factors of higher-order gratings and their phase shift, optical functional devices with higher accuracy and higher performance can be realized inexpensively. More specifically, it is realized by controlling the intensity profile in the longitudinal direction of the radiation mode. Its principle lies in that the structure of the present invention enables controlling the interaction of radiation mode and the guided mode along the waveguide differently.
Representative applications of the invention are DFB lasers and DBR lasers. These devices usually use fine 1st-gratings, and the use of higher-order gratings makes their fabrication easier. Additionally, increase of threshold currents by radiation modes is less. Further, difference in gain from the other longitudinal modes, which exhibits the single longitudinal mode performance, can be made larger than that of 1st-order gratings. In a particularly concrete structure of DFB laser, the phase shift is provided at the center of the waveguide, and asymmetric gratings with gentle inclinations facing to the phase shift are formed between phase shift. In this laser, a low threshold value and a good single mode selectivity can be obtained by sufficiently controlling radiation modes.
When gratings having asymmetric cross-sectional configuration inverted from the former asymmetric gratings are provided, the profile of a radiation mode as an output can be optimized also in a surface emitting laser.
Further, the invention ensures easy and reliable fabrication of asymmetric gratings.
Moreover, even when the gratings are grooved on a side plane of the waveguide stripe, various similar effects are obtained. Additionally, asymmetric gratings can be made very easily in this structure.
Furthermore, when those inexpensive, high-performance optical functional devices are used, optical communication systems are improved in cost and performance.